Heating with Air Conditioning in Winter: Does It Work When It’s Cold?

How air conditioning heats in the cold

The wall-mounted split, multi-split, ceiling cassette and ducted air conditioning we fit all heat as well as cool. In heating mode each one runs as an air-to-air heat pump: rather than turning electricity into heat, it uses electricity to carry heat that is already in the outdoor air into your rooms.

That works in winter because “cold” air still holds heat. The refrigerant inside the outdoor unit is pumped round far colder than the air around it, even on a frosty night, so it keeps soaking up heat. The compressor then squeezes the refrigerant until it is hotter than your room, and the indoor unit blows room air across it. For the whole cycle step by step, see how air-to-air heat pumps work.

Moving heat takes much less energy than making it. GOV.UK’s guidance puts air-to-air systems at about three units of heat for every unit of electricity they use. A plug-in heater gives one.

Does it heat at 0 °C and below?

Yes. Every system has a published operating range: the outdoor temperatures it is designed to heat in. Mitsubishi Electric guarantees its MSZ-LN wall-mounted range to heat down to −15 °C, and Daikin’s UK data sheet for its Perfera wall-mounted units gives −20 °C or −21 °C, depending on size. Other manufacturers publish similar figures, and the one for your model is on its data sheet.

Two things do change as the temperature drops. The first is efficiency: the colder the outdoor air, the harder the system has to work to lift its heat to room temperature, so each unit of electricity buys less heat.

Illustrative figures for a modern wall-mounted system. The real ratio depends on the model, how it is sized and installed, and how it is used.

The second is output. The most heat a system can deliver falls as the outdoor air gets colder, at exactly the time your room needs the most. That is why we size a system at the survey for a cold day in your room, not an average one. It matters most in bigger, older houses: in government modelling published in July 2026, the one home of five where an air-to-air system could not always hold the living room at its target temperature in the coldest week was a four-bedroom detached house built between 1919 and 1944.

Defrost: why the unit stops now and then

In heating mode the outdoor coil runs colder than the air around it. In damp weather around and below freezing, moisture in the air freezes onto it as frost, and frost blocks the air the coil needs. When the unit senses this, it starts a defrost by itself: it briefly reverses, sending warm refrigerant through the outdoor coil to melt the frost, then goes back to heating.

During a defrost you may notice that:

• the indoor unit stops blowing, or blows very gently, so it doesn’t push cold air into the room;
• the outdoor unit’s fan stops;
• water runs off the outdoor unit, and steam rises from it as the frost melts.

None of that is a fault. Vaillant puts a typical defrost at 5 to 15 minutes, depending on how much frost there is, and Mitsubishi Electric’s manual for its MSZ-LN range says that if you switch on during a defrost, it can take a few minutes, ten at most, before warm air comes out. In cold, humid spells it may need to defrost more often.

Don’t switch the system off mid-defrost; that leaves the ice where it is. What isn’t normal is an outdoor unit that stays encased in ice, or one that keeps defrosting on a mild day. If you see either, give us a call.

What it costs to heat a room this winter

The figures below use the Ofgem price cap for 1 October to 31 December 2026, paying by direct debit, as a Great Britain average: 26.32p per kWh of electricity and 7.97p per kWh of gas. Ofgem notes there is no VAT on electricity from 1 October 2026 to 31 March 2027; the gas rate includes 5% VAT. A kWh is the “unit” on your bill.

What a unit of heat costs depends on how much heat each unit you pay for gives you:

Since April 2018, gas boilers fitted in English homes must be rated at least 92% efficient; we use 90% to allow for older boilers and real-world running.

Now take one room on a cold day that needs 6 kWh of heat, about 1 kW for six hours:

Over thirty days like that, that comes to £47.38 for the heater, £15.79 for air conditioning and £15.94 for gas. In a run of hard frosts, at about 2.5 units of heat per unit, air conditioning needs 2.4 kWh a day instead: about 63p a day, or £18.95 over the thirty days.

So against plug-in heaters, air conditioning wins clearly, as we set out in air conditioning vs electric heaters. Against gas, at today’s prices, a unit of heat costs about the same either way. The difference is that air conditioning heats the room you are in, where central heating usually warms the whole house. For gas, electric and air conditioning side by side, see our heating comparison.

These are unit costs only. Standing charges stay the same whichever you use, and the cap changes every three months, so the pence will move; the ratios won’t.

Settings that save money

• Choose Heat, not Auto. In Auto, the system picks heating or cooling for itself. Mitsubishi Electric’s manual, for example, describes switching to cooling once the room has sat about 1 °C above the set temperature for about 15 minutes, so winter sun or a lit fire can have it cooling a room you will pay to warm again later. Daikin’s own advice is to set Heat mode on days like that.
• Pick a temperature and leave it. Daikin advises keeping a constant, comfortable temperature for the best efficiency, rather than turning it up and down for a quick burst of heat.
• Use the night or eco setting instead of off. Many remotes have a night mode, or a saved lower setting such as Mitsubishi Electric’s i-save, that drops the temperature a little while you sleep or are out. Switched fully off, the room and its walls cool right down and the system then has to haul them back from cold.
• Let the timer do the early start. A weekly timer can bring the heating on half an hour before you get up, rather than you turning it up high when you’re already cold.
• Keep doors and windows closed. Daikin’s advice is to keep a stable temperature and avoid trying to heat the outside: close the windows, and the doors to the rest of the house.
• Clean the indoor filters. Daikin recommends cleaning the dust filters regularly, so air flows freely through the indoor unit.

Keep the outdoor unit clear

The outdoor unit is where the heat comes from, so it needs air moving freely through it all winter.

• Clear snow and leaves from around and on top of it, by hand. Daikin and Mitsubishi Electric both advise keeping it clear.
• Don’t cover it or box it in. A cover blocks the air the coil needs and gets in the way of defrosting; a decorative box or a pile of logs does the same.
• Leave room for the water to go. Each defrost leaves water that has to drain away, so keep the space under the unit clear, and don’t stand anything on top of it either. If water pools and freezes in front of it, let us know.

When we fit a system, we place the outdoor unit with the clearances its installation manual asks for, and Daikin advises mounting it off the ground where snow settles.

A winter service

Winter is when air conditioning works hardest at heating, so it is a good time to have it looked at. An annual service keeps it running efficiently and supports the manufacturer warranty. Our engineers clean the filters, inspect and test the system, check it is heating as it should, and leave you a report with anything that needs attention.

See what we cover on our air conditioning servicing and repair page, and see your service price on our price page.

Frequently asked questions

Sources

• Ofgem: Energy price cap unit rates and standing charges (1 October to 31 December 2026)
• GOV.UK: Check if a heat pump is suitable for your home (air-to-air)
• Department for Energy Security and Net Zero: The suitability of reversible air-to-air heat pumps for UK domestic use (July 2026)
• Mitsubishi Electric: MSZ-LN wall-mounted range, guaranteed operating range
• Mitsubishi Electric UK: MSZ-LN service manual (defrosting, Auto mode, i-save)
• Daikin UK: FTXM-R Perfera wall-mounted data sheet (heating operating range)
• Daikin UK: How to use your air-to-air heat pump efficiently
• Daikin: The outdoor unit discharges water or steam
• Vaillant: Defrost cycle of a heat pump
• Mitsubishi Electric UK: Heat pump FAQs (clearing leaves and snow)
• Selectra: Covering your heat pump for winter is a mistake
• Warwick District Council: Air source heat pumps, how to heat your home
• GOV.UK: Boiler Plus factsheet (92% minimum boiler efficiency)

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